High density microfluidic devices are useful in a wide range of research, diagnostic and synthetic applications, including immunoassays, nucleic acid amplification and genomic analysis, cell separation and manipulation, and synthesis of radionuclides, organic molecules, and biomolecules. The advantages of microfluidic devices include conservation of reagents and samples, high density and throughput of sample analysis or synthesis, fluidic precision and accuracy, and a space reduction accompanying the replacement of counterpart equipment operating at the macrofluidic scale.
However, the manipulation of fluid volumes on the order of nanoliters and picoliters has required many new discoveries and design innovations. There are fundamental differences between the physical properties of fluids moving in large channels and those traveling through micrometer-scale channels. See, e.g., Squires and Quake, 2005, Rev. Mod. Phys. 77, 977-1026; Stone et al., 2004, Annu. Rev. Fluid Mech. 36:381-411; and Beebe et al., 2002, Ann. Rev. Biomed. Eng. 4:261-86. For example, at a microfluidic scale the Reynolds number is extremely small, reflecting a difference in the ratio of inertial to viscous forces compared to fluids at macroscale. Fluids flowing in microfluidic systems exhibit reduced turbulence, electro-osmotic and laminar flow properties, and in other ways behave differently than observed at a macroscale. There remains a need for new approaches to effecting efficient flow, containment and mixing of microfluids.